As readers skilled in the art will recognize, this invention has a wide range of application. The following example of aeration is for illustrative purposes only, and does not suggest that the invention is restricted to this application. Details of the separation vessel may vary; however, the essence of the invention and the underlying methodology remains constant: using turbulent circular flow inside the porous tube to sheer off small bubbles of gas to maximize contact surface and enhance the process of dissolving gas into a liquid.
Aeration plays important roles in many industries where process efficiency depends on a concentration of oxygen in the processed liquid; i.e. Brewing, Environmental services: waste water treatment, Farming and Fishery, or Mineral Processing.
Traditional methods of creating conditions for aeration include the use of simple aerated tanks, spray towers, bubble tray columns, and packed columns to create a gas-liquid interface. Traditional technology uses counter current, multiple stages for the gas to be absorbed. While these traditional methods and associated apparatus do achieve aeration, they are inefficient, requiring long processing times and hence large equipment volumes. The inefficiency associated with the traditional prior art approaches arises largely from the relatively low gas-liquid interfacial area to volumes provided by the equipment.
It has been suggested that improved aeration performance may be achieved through the use of an air-sparged hydrocyclone similar to designs used in the mineral processing industry for separation of solid particles from an aqueous suspension. Examples of particle separation methods and apparatus may be found in U.S. Pat. Nos. 4,279,743; 4,397,741; 4,399,027; 4,744,890; 4,838,434; and 4,997,549; 5,192,423. Each of these references is specifically based upon the concept of passing bubbles of air through a suspension of solid particles so that hydrophobic particles attach to air bubbles and form a cohesive froth that may be removed from the separation vessel. The apparatus design are concerned with the creation of gas-liquid contact conditions favorable for efficient particle to bubble interaction and separation with mass transfer.
In addition, various methods of and apparatus for removing volatile content (VCs) from water and other liquids have been known and used in the prior art for a number of years. One of the traditional approaches, generally referred to as “air stripping”, removes VCs from a contaminated liquid by passing a stream of clean air or other gas through the water or other liquid so that VCs transfer from the liquid to the gas and may be removed from the system with the exiting gas. Examples of using swirling motion and a porous tube for such approach can be found in U.S. Pat. Nos. 5,529,701, 5,531,904—Grisham T. et al. The operating parameters of the method described by Grisham are selected to optimize the overall efficiency of both mass transfer between gas dissolved in the liquid phase and gas passing through the liquid. The flow rate of liquid in the Grisham method needs to be set to produce centrifugal force fields with radial accelerations between 400 Gs up to about 1500 Gs compared to accelerations of about 70 G used for particle separation.
In general, the method described by Grisham, dynamically mixes gas bubbles with liquid (thereby rapidly replenishing the supply of molecules of the transferring component in immediate proximity to the gas-liquid interface and minimizing mass diffusion limitations on transfer rate), optimizes the contact time between bubbles and liquid (thereby allowing material transfer to reach or closely approach equilibrium), and cleanly separates post-contact gas and liquid streams (thereby minimizing regressive transfer). The Grisham's objective is to maximize gas velocity flowing through the liquid and diverting both phases (liquid and gas) at the apparatus exit. If a large volume of gas passes through the unit of liquid then mass transfer of gas dissolved in liquid into passing gas is maximized increasing overall gas stripping efficiency. Grisham apparatus works in the regime of very high Gs promoting movement of gas from liquid to gas—but not in reverse.
It is generally assumed that diffusion of gas across an interfacial contact area is instantaneous, but the actual rate of transfer is subject to various limiting factors. In most cases the rate of gas diffusion into the liquid is always favored by maximizing the interfacial area relative to liquid and gas volumes, which means that the key is in generating very small diameter bubbles with narrow size distribution. When very small bubble size and narrow size distribution is achieved then a high gas to liquid volume ratio is achieved. The smaller the bubble, the bigger the gas volume that can be packed into the unit volume with a correspondingly larger surface area. The liquid occupies only voids between highly packed gas bubble spheres and a ratio of up to 50:1 gas to liquid can be achieved. The process of generating bubbles is dynamic and equilibrium must be achieved between creation of new bubbles and bubbles coalescing into bigger ones. The time of interaction must be maximized which puts limitations on the gas velocity.
Inventive discoveries related to the present invention include that —optimum bubble size distribution can be only achieved if a porous tubular housing with mean pores size—below 100 microns is used for the gas diffuser. It has also been discovered that optimum conditions exist for a given range of G forces and Reynolds number for turbulence of flow, which impart limitations on flow rates and the diameter of diffuser. High Reynolds numbers promote maintaining small bubble size and so prevent bubble coalescence by ripping apart all bubbles bigger than eddies in the flow. However, too high G force quickly moves bubbles to the centre due to the buoyancy of the bubbles. Once at the centre, the flow becomes coaxial with consequently drastically lower Reynolds number and bubbles coalescence that rapidly lowers interfacial contact surface area.